Go to the outside unit and in the middle of the model number there will be a 18, 24, 30, 36, 42, 48, or 60. Divide 12 by that number and that'll be your tonnage. It's 102 here and mine is staying at 72 degrees and cycling properly. That 20 degree rule of thumb is just like the sq in of port per cube rule of thumb. If the system is sized right, the ductwork is put together properly, the duct is properly routed and sized, etc that rule of thumb can be thrown out the window.